Enantiospecific dehydrogenation

The principle pathways of lignan biosynthesis in Forsythia have been elucidated from studies using labelled precursors and cell free extracts. The sequence from coniferyl alcohol (9) to (+)-pinoresinol (10), (+)-lariciresinol (11), (-)-secoisolariciresinol (12) and (-)-matairesinol (13) is now clearly established (scheme 1) [30-33]. Lewis et al. have shown that (+)-pinoresinol (10) is formed via direct stereoselective coupling of the two coniferyl alcohol molecules, a process which requires the complimentary action of a specific protein and an auxiliary oxidase or peroxidase [34,35]. (+)-Lariciresinol (11) and (-)-secoisolariciresinol (12) are then formed by consecutive enantiospecific reduction [36], and (-)-secoisolariciresinol is further metabolised into (-)-matairesinol (13) via enantiospecific dehydrogenation, and into (-)-arctigenin (14) via regioselective 0-methylation [37]. 

The next conversion to be demonstrated was the stereo- and enantiospecific dehydrogenation of (-)-secoisolariciresinol 76b into (-)-matairesinol 36b, in a conversion requiring NADP as cofactor (79, 80) the final step leading to (-)-arctigenin 74 involves regiospecific methylation (81). 

C-H activation remains an important topic for catalysis even after thirty years of intensive research. The potential shortcuts it offers for many present routes to a wide variety of chemicals that are produced will continue to inspire industrial and academic research [32], An interesting example involves the enantiospecific, coordination-directed C-H bond functionalisation in the synthesis of a natural product, rhazinilam, an anti-tumor agent. The resulting vinyl moiety obtained in the dehydrogenation was subsequently carbonylated to form a cyclic amide [33],... 

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